1. Field of the Invention
The present invention relates to the control and operation of oscillating circuits using a charge pump, and more particularly, the present invention relates to a charge pump that improves linearity within a circuit coupled to an oscillating circuit that supplies an offset current.
2. Description of the Related Art
Oscillating circuits, such as voltage controlled oscillators, can operate at certain frequencies. The control circuits for oscillating circuits may include a charge pump and a phase shift circuit. The phase shift circuit may be implemented in a radio frequency (RF) transceiver as a phase locked loop. A voltage controlled oscillator (VCO) may be used to implement the phase locked loop and to produce an oscillation from a reference frequency.
Phase locked loops may be used in wireless communication and direct communication devices. Data may be modulated onto at least one RF carrier frequency and transmitted as a modulated signal by a transmitter. A receiver may receive the RF modulated signal, and demodulates it to recapture the data.
In conventional communication devices, the antenna section receives RF modulated signals and provides them to the filtering section. The filtering section passes the RF signals of interest to a low noise amplifier. The low noise amplifier amplifies the received RF signals of interest and provides them as amplified signals to an intermediate frequency (IF) stage. The IF stage may include one or more oscillators.
In many applications, the output frequency of a basic phase locked loop should be adjustable. Within these types of phase locked loops, linearity of the components that generate the output signals is desirable. Non-linearity introduced into the components may cause distortion, or create spurs and raise the noise floor. By raising the noise floor, additional noise is introduced into the loop, which in turn degrades performance. Adjustability of the output frequency also is degraded such that performance of the communication device, wireless or direct, is less efficient in locking to the desired frequency or requires additional resources to reduce noise.